add argMax aggregation

CHANGELOG.md

@@ -1,4 +1,5 @@
 ### 1.5.0
+- feat: #134: add `argMax` aggregation https://clickhouse.com/docs/sql-reference/aggregate-functions/reference/argmax
 - feat: #133: Fix simultaneous queries error when iteration is interrupted
 - feat: #130: Add `distributed_migrations` database setting to support distributed migration queries.
 - feat: #129: Add `toYearWeek` datetime functionality

clickhouse_backend/models/aggregates.py

@@ -1,4 +1,4 @@
-from django.db.models import aggregates
+from django.db.models import aggregates, fields
 from django.db.models.expressions import Star
 
 from clickhouse_backend.models.fields import UInt64Field
@@ -67,3 +67,24 @@ class uniqTheta(uniq):
 
 class anyLast(Aggregate):
     pass
+
+
+class ArgMax(Aggregate):
+    function = "argMax"
+    name = "ArgMax"
+    arity = 2
+
+    def __init__(self, value_expr, order_by_expr, **extra):
+        if "output_field" not in extra:
+            # Infer output_field from value_expr
+            if hasattr(value_expr, "output_field"):
+                extra["output_field"] = value_expr.output_field
+            else:
+                # Fallback: assume CharField
+                extra["output_field"] = fields.CharField()
+        expressions = [value_expr, order_by_expr]
+        super().__init__(*expressions, **extra)
+
+    def as_sql(self, compiler, connection, **extra_context):
+        self.extra["template"] = "%(function)s(%(expressions)s)"
+        return super().as_sql(compiler, connection, **extra_context)

tests/aggregates/tests.py

@@ -10,6 +10,7 @@
     uniqHLL12,
     uniqTheta,
 )
+from clickhouse_backend.models.aggregates import ArgMax
 
 from .models import WatchSeries
 
@@ -170,6 +171,20 @@ def setUpTestData(cls):
         # Use bulk_create to insert the list of objects in a single query
         WatchSeries.objects.bulk_create(watch_series_list)
 
+    def test_argMax(self):
+        result = (
+            WatchSeries.objects.values("show")
+            .annotate(episode=ArgMax("episode", "date_id"))
+            .order_by("show")
+        )
+
+        expected_result = [
+            {"show": "Bridgerton", "episode": "S1E1"},
+            {"show": "Game of Thrones", "episode": "S1E1"},
+        ]
+
+        self.assertQuerysetEqual(result, expected_result, transform=dict)
+
     def _test_uniq(self, cls_uniq):
         result = (
             WatchSeries.objects.values("show", "episode")